A device known from U.S. Pat. No. 4,170,951 for recognizing missed stitches has a thread force-measuring device with a spring clip, with a wire strain gage element fastened to it, which is arranged on the sewing machine in the path of the needle thread. This element generates an electrical voltage that is proportional to its deformation caused by the deflecting movement of the spring clip. The electrical voltage, which consequently ultimately represents the tensile force in the needle thread ("thread force"), is sent via an amplifier and a low-pass filter to the input of an analog/digital converter in order to form a series of digital measured values from it, which are sent to the signal input of a digital evaluating device.
A first, lower maximum is formed in the course of a stitch during the widening of the needle thread loop, and a second, higher maximum of the tensile force is generated during knotting. To detect a missed stitch, checking is performed to determine whether or not the lower maximum reaches a defined minimum level. To do so, the evaluating device presets a defined time window, whose position corresponds to the expected location of the lower maximum, during each period of revolution of the main shaft. The measured values are compared within the time window with a threshold value in order to signal a missed stitch when the amplitude of the measured values within the time window remains below this threshold value.
The change in the tensile force over time within one period of revolution of the main shaft is not always the same even in the case of trouble-free operation of the sewing machine, but it may change from one case to the next, depending on various external conditions, e.g., the material of the thread or the nature of the fabric to be sewn. Thus, the measured value curve being monitored to recognize missed stitches may differ from one case to the next, so that special precautionary measures are to be taken to guarantee satisfactory recognition of missed stitches despite these differences.
A measure pointing in this direction, which is realized in the above-described prior-art device, consists of standardizing the measured values to be compared with the threshold value within the time window to the base value of the measured value curve by scanning this base value, i.e., the measured value amplitude occurring prior to the expected maximum, within a correspondingly positioned further time window and subtracting it from the measured values to be compared. The threshold value used for the comparison in the prior-art case is also set to a level which depends on the amplitude of the second, higher maximum (e.g., to about 5% of this amplitude). Since the prior-art device contains no special means for detecting this amplitude, it is necessary to use any empirical value.
Only amplitude differences of the thread force curve can be compensated for with the above-mentioned measures. However, there are also differences or variations in the chronological location of the thread force maximum to be monitored. If, for example, the machine speed changes, the entire profile of the thread force curve will be shifted. To take this circumstance into account, a plurality of consecutive measured values are checked within the time window in the prior-art case, and a missed stitch is signaled only if all these measured values are below the threshold set.
However, the changes in the thread force depend on so many parameters and may be so different depending on the particular class of the sewing machine that the above-described precautionary measures may not suffice under certain conditions for reliably detecting missed stitches. In particular, the flexibility and the adaptability of missed stitch recognition is limited by the strict presetting of defined time windows. It is, of course, possible to adjust, besides the threshold level, also the starting point and the width of the actual time window to the course of the thread force that can be expected in the particular case, but this requires skilled workers and a corresponding device. Aside from this, it is not always possible to accurately predict the locations of possible maxima of the thread force curve for all conditions. The actual shape of the thread force curve in the case of a missed stitch is also hardly predictable in many cases. Since missed stitches are very rare compared with normal stitches (the probability of missed stitches is a fraction of one per thousand), it would be necessary to perform relatively long test runs covering many thousand stitches to find out how a missed stitch is manifested in the shape of the curve. This would take much time and material.
A device which recognizes missed stitches under defined conditions on the basis of the thread force signal even without a time window is known from DE-AS 26,06,035. In this device, the total mean value of the thread force signal formed over preceding revolution periods of the main shaft, multiplied by a weighted set value, is selected as the threshold for detecting a supernormal peak value which indicates a missed stitch. However, this functions only if the missed stitch is really manifested as an above-normal absolute peak value of the thread force, i.e., in the special case of "sewing on continuous rows of zippers", for which the prior-art device is expressly designed. However, if a missed stitch is manifested differently, e.g., by the missing of one of several maxima of the thread force curve (e.g., as will be described below), the above-described prior-art "time window-less" method is obviously unfit for use.